You can outrank him. My guess is he has spent some time trying to debunk your product and site, but you should have all of your effort in your own.
Find the keywords he is ranking for and SEO pages on your own site (or another site you build) for those keywords. You can often get multiple page rankings for those keywords and push him off the top page with good SEO.
There really isn't any other way than starting your own reputation management campaign and building a few sites that promote your product and site in a good light. This may seem like a big project, but it sounds to me like it would be worth a couple hundred dollars to build some blogs (10-20) about your site, and run an SEO campaign targeting the keywords he went for.
My guess is he's not doing any kind of overly competitive linking to the bad reviews, so it should be a keyword that you can through some domain, IP, and anchor text diverse links to and outrank him in a couple weeks time.
Hope this helps. I know it sucks, but you can't let someone just come in and take ya down. Fight back.